Patent GB-1389542-A (GB)
Title: Methods of securing a semiconductor body to a support
IPC: H01L21/607
Filed: 1971-06-17
Abstract: 1389542 Welding by pressure; ceramic to metal seals MULLARD Ltd 3 May 1972 [17 June 1971] 28449/71 Headings B3R and B3V [Also in Division H1] A semi-conductor body is bonded to a support by applying a layer of malleable metal having a melting point of 125 C.-330 C. between the body and support and applying a steady pressure to the assembly of 1-5 tons p.s.i. while maintaining the assembly at a temperature of 75- 300 C. and below the melting point of the metal layer and below the lowest temperature at which a liquid phase would form at the facing surfaces, the pressure and temperature being together applied to achieve within a period of not more than 30 seconds a bond having a tensile strength minimally of 20% of the Ultimate Tensile Strength of the metal layer. In bonding a silicon transistor body 39, Fig. 2, plated at 49 with titanium and silver to a copper support 33 brazed to a steel base 31, the support and base being gold plated, a foil 41 of lead, silver and tin is arranged between the body 39 and support 33 and pressure of 2À5 tons p.s.i. is applied to the assembly for 10 seconds while the temperature is at 240 C. In another embodiment a silicon transistor body coated with gold is bonded to a gold plated iron base having iron lead-in wires, using a foil of 60/40 tin/lead solder. Alternatively a 37/63 cadmium/tin solder foil may be used. In further embodiments a large scale silicon integrated circuit wafer 71, Fig. 5, coated with titanium and silver is bonded to a gold plated Kovar (Registered Trade Mark) base 65 of a multi-lead envelope using a foil 72 of lead, silver, tin and four silicon transistor bodies plated with titanium and silver are bonded to the upper surface of a beryllia disc using foils of lead silver, tin alloy, the beryllia disc being coated on its lower surface with silver and palladium and pressure bonded to a copper support using a solder foil intermediate layer. In another embodiment; three silicon transistor elements 102, Fig. 7, coated with titanium and then gold are simultaneously bonded to gold pad portions 101 on a beryllia member 94 using foils 103 of 72/28 lead/indium solder. Gold plated nickel lead out strips 97, 99 are bonded to further gold pad portions 99 using soft solder foils 100 and the beryllia body 94, gold plated at 96 is pressure bonded to a copper and gold plated copper plinth 92 using a soft solder foil 95. The plinth 92 is also pressure bonded to a copper base 91 coated with nickel and gold using a soft solder foil 93. Passive circuit elements are secured to the beryllia member using adhesive and connected to the gold pattern after which the device is encapsulated in silicon plastics material. A press in which the bonding may be effected comprises a fixed steel base 3, Fig. 1, heated by an electric element 6 and having a support base 9 in which a thermocouple is inserted. A protective gas, 10% hydrogen in nitrogen is supplied via an aperture 12 in an asbestos cover 8 about the work. An upper movable head 2 comprises a steel pressure plate 21 heated by an element 24, a rubber shock absorber pad 18 being sandwiched between an insert 15 on the press head 2 and a support 20 for the plate 21. The intermediate solder layer may be a coating rather than a foil and may be of indium or lead cadmium tin alloy. The substrate may be of iron, nickel, cobalt alloy coated with silver or nickel, of steel coated with gold, of uncoated copper, of molybdenum or of aluminia or zirconia. Ceramic substrates may be metal plated or unplated if used with indium as the intermediate member. Beryllia ceramic as a substrate may be plated with gold, palladium alloy. The semi-conductor body may be germanium or gallium arsenide, the latter coated with tin followed by gold.